Generally, in semiconductor laser devices, an insulating film called coating film is attached to opposite facets of a resonator thereof for the purposes of reducing a working current, preventing optical feedback, creating high-power outputs and the like as disclosed in the patent publication of JP-3080312 or the laid open patent publications JP-A-2002-100830, JP-A-2003-101126 and JP-A-2004-296903.
Especially, in semiconductor laser devices demanded with high-power outputs, a coating film of low reflectance is formed at a front facet side (emission facet side), and a coating film of high reflectance is formed at a rear facet side, thereby creating high-power outputs. The reflectance of the rear facet coating film is not smaller than 60%, preferably not smaller than 80%. The reflectance of the front facet coating film may not be merely low, and its value is selected depending on the characteristics required for a semiconductor laser device. For instance, the reflectance is selected to be about 0.01-3% for semiconductor laser devices for fiber amplifier excitation used in combination with fiber gratings, about 3-7% for ordinary high-power semiconductor laser devices, and about 7-10% for the case where a measure for optical feedback is necessary.
In a high-power bluish purple semiconductor laser device of 50 mW or over using a GaN substrate, the reflectance at the emission front facet should have a value of about 5%-15%. If it is intended to obtain a reflectance of 6%, the reflectance control required is at 6±1%. In general, the reflectance at a front facet from which a laser beam is emitted in a semiconductor laser device is controlled by the thickness and refractive index of a single-layered dielectric film, e.g. by the thickness and refractive index of a dielectric film such as of Al2O3, SiO2 or the like.
In FIG. 25, there is shown a construction view of a conventional semiconductor laser device whose oscillation wavelength is 405 nm. In the figure, the semiconductor laser comprises a GaN substrate 101, an active layer 102, upper and lower clad layers 103, an electrode 104, a low reflection film 112 formed at a laser front facet, and a high reflection film 107 formed at a laser rear facet. A laser beam 105 is emitted. It is usual that, for the low reflection film used at the laser front facet, there is used a single-layered film having such an optical thickness as to provide an integral multiple of λ/4±α (the reflectance is controlled by α). At the front facet of the semiconductor laser, the density of a laser beam is so high that the temperature is likely to rise, under which this low reflection film plays a role as a heat dissipating plate (heat spreader). Accordingly, a 3λ/4±α film of aluminum oxide is ordinarily used.
In general, the reflectance is calculated according to a matrix method using, as parameters, a refractive index of a substrate, a coating film thickness formed on the substrate and a refractive index thereof, and a free space (usually, air with a refractive index of 1).
In FIG. 26, the wavelength dependence of reflectance is shown in the case where an aluminum oxide film (refractive index: 1.664) whose α is set at 21.5 nm (film thickness: 204 nm) is disposed at a front facet of a bluish purple semiconductor laser device (refractive index of GaN substrate: 2.5) whose oscillation wavelength is at 405 nm. In FIG. 27, the thickness dependence is shown. From FIG. 27, it will be seen that in order to realize 6±1%, the film thickness has to be controlled at an accuracy of ±1% relative to a designed value of 204 nm. In this way, in the bluish purple semiconductor laser whose wavelength is as short as 405 nm, the coating film thickness becomes thinner, correspondingly to the wavelength ratio, than those of conventional lasers for DVD in the 680 nm band and for CD in the 780 nm band, thus requiring more precise control of the film thickness. Thus, when using film-forming techniques whose thickness control is at a level of about ±5%, such as vacuum deposition, sputtering and the like, a difficulty is involved in reflectance control, thus inviting a lowering of yield.
In the case where, for example, a reflectance of 6±1% is realized using conventional semiconductor laser devices having such constructions as set out above, it is necessary to suppress a variation of film thickness within ±1% for such an aluminum oxide single-layered film, with the attendant problem that the reflectance control lowers, thereby leading to degradation of yield. Hence, there is an urgent need to enable one to reliably, reproducibly select a reflectance at an emission facet depending on the purpose of a semiconductor laser.
The invention has been made in view of such problems as stated above. That is, an object of the invention is to obtain a semiconductor laser device wherein a reflectance can be stably controlled irrespective of the variation in thickness and refractive index of a dielectric film serving as a reflection film formed on a facet of a semiconductor laser.
Other objects and advantages of the invention will become apparent from the following description.